1. Field of the Invention
The present invention relates to an image reading device, and more particularly to an original image reading device for optically reading image information from an original document and adapted for use in an image information processing apparatus such as a facsimile apparatus, an image scanner or the like.
2. Related Background Art
Requirements for smaller dimensions, lower weight and lower cost have become stronger in recent years in the field of image information processing apparatuses, such as facsimile apparatus, image scanners and the like. For facsimile apparatuses in particular, there have emerged various requirements for design and function, in addition to the above-mentioned ones, following the expansion of the market including so-called home facsimiles for personal use.
For the original image reading device to be incorporated in such image information processing apparatus for optically reading the image information of the original document, there have been employed a reduction reading system utilizing a line sensor such as a CCD and a reduction optical system; a contact reading system utilizing a line sensor of a length equal to the width of the original, an equal-magnification optical system such as a condensing fiber lens array, and a light source such as an LED array, all integrally assembled in a frame; and a complete contact reading system utilizing a line sensor and a light source integrally assembled in a frame, without the lens array, wherein the line sensor is maintained close to the original for directly reading the original image.
An example of such image reading device is schematically illustrated in FIGS. 1 and 2 which are respectively a cross-sectional view and a schematic perspective view.
There are shown a line sensor 101 for optically reading the image of an original P; an automatic sheet feeding unit 113 consisting of a separating member 111 and a sheet feeding roller 112, for separating and feeding the originals P one by one toward said line sensor 101; and a transport (pressure) roller 102 serving as original contacting means for contacting the fed original P to a reading position 104 of the line sensor 101 and also as original transport means for transporting said original P.
In such an image reading device, a transport (pressure) roller is provided opposite to the reading part of the contact sensor thereby maintaining the original in close contact with the reading line of said contact sensor, in order to prevent image blur at image reading, resulting from eventual lifting of the original from or movement thereof on the reading line of the line sensor.
Said transport (pressure) roller, for maintaining the original in close contact with the reading line of the reading part of the contact sensor, is generally made of a rubber material, in order to attain close contact of the original regardless of certain surface irregularities of said reading part.
Also the surface of said transport (pressure) roller is usually read by the reading part, prior to the reading of the image information of the original, for determining a reference color for image information reading of the original, and, for this purpose, said surface is usually white.
Thus a white rubber roller is generally employed for the transport (pressure) roller mentioned above.
However, the functions of original transport means and original contacting means, if performed by a transport roller in the image reading device, may result in following drawbacks:
(1) In order to maintain the original in close contact with the line sensor for reading the original image by said line sensor, the transport roller has to be as long as the line sensor or the reading width of the original. Therefore the local unevenness in the transporting power results from the planarity of the reading face of the line sensor, eventually causing skewed advancement of the original; PA1 (2) The transportation of the original in contact with the reading face of the line sensor increases the load on a motor for driving the transport roller, thus eventually leading to heat generation or control abnormality in said motor. Therefore, for achieving stable transportation of the original, there is required a large motor, which becomes an obstacle to achieving the goals of cost reduction, compactness and weight reduction of the device; PA1 (3) An original eventually fed in skewed state tends to become more skewed in the course of transportation, as will be explained in the following with reference to FIGS. 3A to 3C.
FIGS. 3A to 3C are schematic plan views of the image reading device shown in FIGS. 1 and 2, wherein the line sensor is not illuminated but is positioned behind the transport roller 102.
When an original P is fed in a skewed state in the automatic sheet feeding unit 113 as shown in FIG. 3A, it is advanced to the transport roller 102, as shown in FIG. 3B, by the transporting force (a) of the automatic sheet feeding unit 113, indicated by an arrow. Then, as a part of the original P is pinched between the transport roller 102 and the line sensor as shown in FIG. 3C, said part is subjected to a transporting force (b) of the transport roller 102, indicated by an arrow. In general, the peripheral speed of the feed roller (not shown in the Figure) of the automatic sheet feeding unit 113 is selected somewhat smaller than that of the transport roller 102. Consequently a load (c) indicated by an arrow is generated in a direction opposite to the transport direction of the original, due to said different in peripheral speed and a load induced by the frictional member, and a rotating force (d), also indicated by an arrow, is generated by the aforementioned transporting force (b) and the load (c), thus causing an increased skew in the original P.
On the other hand, if a pressure roller separate from the transport roller is required, there are required a space above the reading part for positioning said pressure roller and a space for accommodating a drive system, such as gears, for driving said pressure roller in the same driving direction as that of the transport roller, so that the total height of the device inevitably increases.
Also if a pressure roller of a smaller diameter is employed for reducing the total height of the device, the pressure roller shows more bending so that the assembling precision of the reading part and the pressure roller has to be improved in order to securely guide the light, emitted from the light source of said reading part and reflected by the roller surface, to a photoelectric converting device in the reading part. For this reason there will result an increase in the manufacturing cost and a loss in productivity.
Furthermore the pressure roller is required to have a small friction coefficient with the glass of the reading part but a large friction coefficient with the original for ensuring secure transportation, and it is difficult to select a material satisfying such requirements.
As explained in the foregoing, there are still left various technical problems, in order to satisfy the ever increasing requirements for smaller dimension, lower weight and lower cost of the image reading device.